1. Field of the Invention
The invention relates to magnet assemblies particularly for use in magnetic resonance imaging (MRI).
2. Description of the Related Art
Conventional MRI systems are large superconducting systems and generally have to be carefully mounted in special areas. Although this is acceptable for the purposes of laboratory applications such as spectroscopy and human body imaging, it is not so useful for recently developed applications of MRI.
We have realised that a step towards solving this problem would be achieved by providing a magnet assembly based on permanent magnets rather than the usual superconducting magnets which require associated cryostats and the like to achieve a superconducting condition. Permanent magnet structures for magnetic resonance imaging (MRI) have been previously described. The most compact and economical in materials usage are those described by E. Potenziani and H. A. Leupold (IEEE Transactions on Magnetics Mag-22, 1078-1080, 1986). These make use of "cladding magnets" to oppose the magnetomotive force (mmf) around the outside of the structure. This has the effect of preventing flux leakage so as to most effectively use the permanent magnet material and obtain good field uniformity in the working volume.
A disadvantage of this type of structure is that the working volume is substantially enclosed. This means that the magnet must be large enough to accommodate all the object to be examined, and that there are difficulties when the subject is large. Also when it is required to examine a relatively small volume of a large object, this type of "whole-body" magnet may not be the most economical.